Effect of Bond Geometry on Shear Strength and HTS Reliability for Au Ball Bond on Al Pad

The effect of bond geometry on bond strength and reliability is studied for 25-
$\mu \text{m}$
gold wire bonds on standard CMOS aluminum pads. An accelerated method for parameter optimization is used to produce ball bonds with optimized shear strength (SS) and bond geometry. Ball bonds have a diameter of 56.5
$\mu \text{m}$
with a bond height of 12, 16, and 20
$\mu \text{m}$
representing the low, medium, and high bonds, respectively. Low height bonds achieve 126.1 MPa strength and endure up to 41.9% ultrasound (US) before deformation begins, whereas high bonds achieve 109.8 MPa strength and withstand a maximum of 39.8% US. Low bonds achieve higher strengths possibly due to the higher level of US that can be used. Bond reliability is assessed in high-temperature storage tests using a shear test (a destructive method) and piezoresistive stress sensors (a nondestructive method). The bonds are aged at 200 °C for
$\sim 500$
h. Quantifying the reliability performance using shear test results found to be difficult due to poor time resolution. However, from features in the stress sensor signals, a characteristic time (
$t_{c})$
is derived during aging. The
$t_{c}$
value indicates the time it takes for the bonds to severely degrade due to intermetallics and crack formation. The
$t_{c}$
values for low, medium, and high bonds were found to be 199.9 ± 8.7, 225.9 ± 9.5, and 231.9 ± 6.8 h, respectively. The result that suggests ball bonds with higher heights are more reliable than the bonds with lower heights for a constant ball bond diameter. This result is corroborated by the SS before and after aging being the same for the high bonds, whereas SS dropped by 3% for the low bonds. The nondestructive method was demonstrated to have an estimated time resolution better than 10 h, which allows for precise comparisons of the reliability of various processes.